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4) and the Principle of Stationary Phase ([Gough, P. T. (1998)]). The image reconstruction task can, as a result, be done by multiplication of the frequency domain inverse sonar imaging model. 8) The array spacing from Nyquist spatial sampling and classical array theory is λ/2 (two way equivalent 2π phase shift), which means that for angles of arrival of a wave-front the interelement phase difference must be less than 2π ([McHugh, R. et al (1998)]). This is also true for motion errors. To correctly form a synthetic aperture the platform position must be known within 1/8 of a wavelength so the echoes can be coherently combined with negligible image deterioration ([Cutrona, L.
19. Artificial target used for resolution tests. 10. Results To test the system and access its capabilities a series of test missions were performed in the Douro river, Portugal. For the first tests an artificial target was placed in the muddy river bottom and the autonomous boat programmed to make several paths through the area. The artificial target is a half octahedral reflector structure made of aluminium (Fig. 19). It measures 20x20x20cm, but the target response seen by the sonar should be like a point after correct image synthesis.
For the first tests an artificial target was placed in the muddy river bottom and the autonomous boat programmed to make several paths through the area. The artificial target is a half octahedral reflector structure made of aluminium (Fig. 19). It measures 20x20x20cm, but the target response seen by the sonar should be like a point after correct image synthesis. 37 Synthetic Aperture Techniques for Sonar Systems Fig. 20 shows one image of the artificial target obtained though the matrix implementation of the back-projection algorithm as describe previously.